The present invention relates in general to a moisture detecting apparatus for automatically detecting the ambient moisture on the basis of an output signal from an electric resistance moisture sensor and, more particularly, to a moisture controller for automatically controlling the ambient moisture on the basis of an output signal from an electric resistance moisture sensor in order to optimize the ambient moisture in a room and the like.
It has already been known that the actual moisture level is detected using a moisture sensor which varies with the resistance value of itself depending upon a change in ambient moisture. As a moisture sensor, for example, a device is employed which is made of a porous ceramic material and whose resistance value continuously changes due to the water molecule adsorption phenomenon of the porous ceramic. Conventionally, a reference voltage is applied across the terminals of the moisture sensor of the above-mentioned type, thereby obtaining a moisture detection voltage signal responsive to the ambient moisture level. In this case, when a DC power voltage is directly applied to the moisture sensor, the porous ceramic moisture sensor, which adsorbs water molecules, will have made its moisture detection ability worse due to a so-called "electrolytic corrosion" phenomenon. Therefore, an AC voltage is conventionally applied across the terminals of the above-mentioned sensor.
However, in this case, another critical problem is caused whereby the moisture detection accuracy of the moisture sensor may deteriorate. This is because the moisture sensor of the above-mentioned type includes a capacitance component as an equivalent circuit, causing a noise voltage which is due to this capacitance component being added to the moisture detection voltage signal which is obtained on the basis of the applied AC voltage and of the sensor resistance which varies in accordance with the ambient moisture.